1. Technical Field of the Invention
This invention relates to improvements in circuitry for driving polyphase motors, and more particularly to a method and apparatus for relatively rapidly driving a polyphase motor for a disk drive from an inactive state to an operable state for performing a memory access, and still more particularly to a circuit and method for detecting motion of a polyphase motor for a disk drive system.
2. Background of the Invention
Although the present invention pertains to polyphase and/or polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type which are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a xe2x80x9cYxe2x80x9d configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three xe2x80x9cYxe2x80x9d connected coils, connected in three sets of four coils, each physically separated by 90 degrees. In operation, the coils are energized in sequences, in each of which a current path is established through two coils of the xe2x80x9cYxe2x80x9d, with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three phase motor.
In the past, during the operation of a polyphase dc motor for a disk drive system, such as a spindle motor for spinning the disk media upon which data is stored, it has been recognized that maintaining a known position of the rotor of the motor is an important concern. There have been various ways by which this was implemented. The most widely used way, for example, was to start the spindle motor in a known position, then develop information related to the instantaneous or current position of the rotor. One source of such instantaneous position information was developed as a part of the commutation process, and involved identifying the floating coil, and monitoring its back emf, that is, the emf induced into the coil as it moves through the magnetic field provided by the stator.
Oftentimes, a spindle motor of a disk drive system is slowed due to a relatively prolonged absence of requests to access the disk drive. During these periods of inactivity, the spindle motor may be slowed considerably or stopped altogether. Conventional disk drive systems attempted to quickly spin up the spindle motor from an inactive state by initially determining whether the spindle motor was moving. This determination was accomplished by tri-stating the phase windings of the motor for a sufficient period of time to detect at least two consecutive zero crossings of the back emf signals associated with the phase windings. In the event the rotor is spinning very slowly, the amount of time necessary to detect consecutive zero crossings may undesirably approach several hundred milliseconds. Once consecutive zero crossings of the back emf signals are detected, thereby indicating a spinning rotor, a resynchronization procedure is executed to synchronize to the rotor spin the application of drive signals to the motor""s phase windings. In the event successive zero crossings of the back emf signals are not detected even after the lapse of several hundred milliseconds, an inductive sense routine is initiated to determine the position of the rotor, followed by executing a spin-up procedure to bring the spindle motor to the desired operable speed. Consequently, the delay related to returning rotor spin to operable levels may be excessive.
Based upon the foregoing, there is a need for a controller and method for controlling the motor of a disk drive to efficiently increase motor spin to operable spin speeds following periods of disk drive inactivity, and particularly for initially determining whether or not the disk drive motor is spinning.
The present invention overcomes shortcomings in prior and existing disk drive systems and satisfies a significant need for high speed disk drive operation. According to an embodiment of the present invention, a controller for a polyphase motor, such as the spindle motor of a disk drive, initially senses current on a single selected phase winding of the motor. A period of time is measured until the sensed current reaches and/or surpasses a predetermined threshold current level. The measured period of time is then selectively stored in memory. Current is sensed on the selected phase winding a number of times and time period measurements are taken corresponding thereto. Minimum and maximum measured time periods are thereafter compared with each other. The rotor of the disk drive motor is determined to be spinning if the minimum measured time period and the maximum measured time period are not within a predetermined percentage of each other. Alternatively, the rotor of the disk drive motor is determined to be stopped if the minimum measured time period and the maximum measured time period are within the predetermined percentage of each other.
Having determined whether or not the rotor of the motor is spinning, the controller is capable of relatively quickly bringing the motor to an operable speed for performing a memory access operation. Specifically, in the event the rotor is determined to be spinning, the controller then executes a resynchronization operation to synchronize the application of the drive signals to the already spinning rotor. In the event the controller determines that the motor""s rotor is not spinning, a spin-up operation is executed by the controller to spin up the motor from the inactive state to an operable state. Because the time to execute the current sensing operations is substantially less than the time to execute the initial resynchronization operation in prior systems for a slowly moving rotor, the time to bring the rotor up to a spin level to suitably access the disk is markedly reduced.